The present disclosure is related generally to the field of vibrators used as seismic energy sources. More specifically, the present disclosure is related to vibrators having capability to suppress airwaves generated by the operation of such vibrators.
Seismic surveys for oil and gas exploration commonly use seismic vibrators to generate seismic energy that is transmitted into the Earth's subsurface. Airwaves (also often referred to as “airblast” or “air-coupled” waves, hereinafter “airwaves”) are coherent noise trains produced by a surface seismic source, propagating at the speed of sound in air. Airwaves may be entirely coupled with the air, or they may be partially coupled with the near surface if the phase velocity of Rayleigh waves and the speed of sound in air are the same. Seismic vibrators usually operate above the ground surface, with vibrational energy transmitted into the subsurface through a baseplate resting on the ground surface. In such seismic surveys, it is common to make use of a vibrator mounted on a truck. Because the majority of the vibrator is exposed to the air, including the upper surface of the baseplate, some of the vibrational energy during operation is transmitted through the air as sound waves.
Such air-coupled sound waves are often of sufficient intensity to detrimentally affect the measurement of seismic signals of interest, specifically those seismic signals reflected from acoustic impedance boundaries in the subsurface. The reflected seismic signals are small in magnitude and waves propagating through the air may cause slight vibrations of seismic sensors (typically geophones or accelerometers) deployed proximate the ground surface, or vibrations of the ground itself. Such vibrations are typically of relatively high amplitude, and may result in such air-coupled waves being detected by the seismic sensors and recorded. Because air waves can cause the ground itself to vibrate, burial or shielding of the seismic sensors often times fails to adequately address the problem.
Airwave noise is strongest at higher frequencies, typically 30 Hz and above. The actual value of the frequency range at which air-wave coupled Rayleigh waves are more energetic depends on the elastic properties of formations proximate the Earth's surface.
Signal processing and hardware techniques have been used to attenuate the effects of airwaves on detected seismic signals. Signal processing techniques for removing Rayleigh waves (typically having a frequency less than 15 Hz.) have proven ineffective at the frequencies associated with airwaves because typical seismic sensor spacing results in spatial aliasing. Spatial aliasing of surface waves can be mitigated using point receiver acquisition, as contrasted with the typical practice of summing signals from subsets of the seismic sensors to attenuate the effects of near surface propagating seismic waves, but because of low propagation velocity of typical airwaves, the higher and more energetic frequencies thereof tend to remain spatially aliased.
It is desirable to have an improved method and apparatus for reducing the effects of airwaves on seismic signals detected from a vibrator-type seismic energy source.